The transistors you mention are custom 2SB1620 and 2SD2488, made originally for Sony by Sanken. I haven't had any luck tracking down any data sheets, unfortunately. Someone else here might know better. Most japanese semiconductor manufacturers are a pain once they've discontinued a product, they like to pretend it never existed.

I dunno, from what I hear Sanken make pretty bloody good transistors. If you'd rather not risk messing with them, you could easily sell them on here for a pretty penny, I'm quite willing to bet plenty of people would offer to buy them.

As mentioned, darlingtons have the driver inside. One big drawback is the driver is the same temperature as the output. This makes a temperature compensation circuit (servo) an absolute must, well with class AB applications. But they work basically the same.